This invention relates to graphical data apparatus and, more particularly, to an apparatus for determining the position of a movable element in a data area.
Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element (such as a stylus or cursor) and transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. One model of such type of digitizer, called a "GRAPHBAR", employs a pair of "point" microphones, having generally circular receptivity patterns, mounted in spaced relation in an elongated generally rectangular housing. The housing or "bar" can be conveniently moved to a position adjacent an area in which the position of a movable element, containing a sound source, is to be digitized. The transit time of sound traveling from the source to each microphone is used, in conjunction with the speed of sound in air and known geometrical relationships, to compute the position of the movable element.
In the described type of digitizer there is a region adjacent the location of the transducers, sometimes referred to as a "dead space", where it is difficult to determine the position of the movable element with sufficient accuracy. This is illustrated in conjunction with FIG. 1 which shows a sonic digitizer that includes a bar 90 in which are mounted a pair of spaced apart microphones 51 and 52. The microphones are mounted near opposite ends of the bar and facing the area 10 to be digitized. [The size and shape of the area 10 is somewhat arbitrary and depends, inter alia, upon the necessary accuracy of the digitizer readings.]The x and y directions are as shown by the axes 59 in the diagram. Consider the points 1 and 2, which are a distance d apart and at respective distances L.sub.1 and L.sub.2 from microphone 51, and the points 3 and 4 which are also a distance d apart and at respective distances L.sub.3 and L.sub.4 from microphone 51. Geometrical considerations dictate that the difference L.sub.4 -L.sub.3 will be greater than the distance L.sub.2 -L.sub.1 This makes it more difficult to accurately determine the y coordinate location of points near the bar 50. Therefore, a "dead space" 11 [whose specific size and shape are determined by desired accuracy]is typically marked off and not used as part of the area in which the position of the movable element is to be accurately located. The "dead space" can be employed for a function such as menu selection, if needed, which does not require high accuracy in two dimensions.
In addition to the "dead space" being wasted in many applications, the need to mark it off is a nuisance, and the risk of inaccurate measurements in the "dead space", if the movable element enters this area, is problematic.
It is among the objects of the present invention to reduce the problems associated with digitizer "dead space" and to generally improve the efficiency and compactness of digitizer equipment.